The present invention relates to intrusion detection systems of the type employing two detection technologies in one housing. More particularly, the invention relates to a supervision circuit to enable a user to determine the malfunctioning of either one of the technologies employed.
The prior art is replete with a number of systems which essentially are referred to as dual detection intruder systems. These systems utilize a pair of intruder detection subsystems, each functioning to detect intrusion by a technology different from the other. Upon the receipt of an alarm from both systems the user is warned of a valid intrusion.
In regard to such systems there have been many approaches which utilize combination systems such as microwave and passive infra-red, as well as other combinations. See for example U.S. Pat. No. 4,660,024 entitled "Dual Technology Intruder Detection Systems" by R. L. McMaster, issued on Apr. 21, 1987. In that patent there is shown the use of a dual technology intrusion detection system employing microwave and a passive infra-red (PIR) subsystem, both of which will provide an output upon the occurrence of a valid intrusion.
U.S. Pat. No. 3,801,978 issued on Apr. 2, 1974, entitled "Ultrasonic Microwave Doppler Intrusion Alarm System" to D. N. Gershberg et al. depicts a dual technology system which employs an ultrasonic and microwave subsystem in combination.
U.S. Pat. No. 4,625,199 issued on Nov. 25, 1986, to M. M. J. Pantus and entitled "Combination Intrusion Detector System Having Correlated Ultrasonic and Microwave Detection Sub-Systems, describes still another system utilizing ultrasonic and microwave systems in combination.
Thus it is clear that to provide intrusion detection systems with reliability the prior art has understood the need to utilize or combine two or more technologies in a common intruder detection system. See for example U.S. Pat. Nos. 3,725,888, 3,801,978, 4,243,979, 4,275,390, 4,331,952 and 4,401,976. These proposals go back many years but recently have received widespread use due to the fact that the cost of electronics has reached the level that enabled the commercialization of such systems.
In these systems the outputs of the different intruder detection subsystems, as for example microwave and passive infra-red subsystems, are fed to an output AND gate or its equivalent. In the event that the outputs of both subsystems indicate that an intrusion has been detected substantially simultaneously or within a given time interval then the AND gate provides the alarm activating signal. The advantage of such a system is that false alarms will only occur on relatively rare occasions where a spurious or false alarm-producing event is detected by both subsystems at about the same time. Hence, by combining diverse technologies the probability of false alarming is minimized.
In any event, as is known, a major drawback of a combination system resides in the fact that if one of the sensors or subsystems fails to operate properly the integrity of the entire system can be defeated. The prior art was cognizant of such problems and reference is made to U.S. Pat. No. 4,710,750 issued on Dec. 1, 1987, entitled "Fault Detecting Intrusion Detection Device" to R. A. Johnson.
In that patent there is described an improved intrusion detection system of the dual sensor type wherein one sensor is a PIR sensor and the other is a microwave sensor. The improvement comprises counting the detection of intrusion separately by the microwave sensor and by the passive infra-red sensor. Thereafter the counts by the two separate systems are compared and an indication is given if the number exceeds a certain user-selectable threshold to indicate a fault in one of the two sensor systems.
In any event, as one can ascertain from the abovenoted patent, the difficulty that one can experience with this type of result is that the logic control means, which receives the outputs of the first and second systems, operates to compare numbers stored therein and outputs a false signal in response to this comparison. Hence, the comparison circuitry is relatively expensive and operates to provide user-selectable ratio numbers along input lines which ratio numbers are a function of the ratio of the microwave output with respect to the PIR output and vice verse. Hence, as indicated, this system is relatively expensive to produce and implement.
Certain of the prior art systems, as above described, also operate to determine faults in each of the dual technology systems in order to also indicate an alarm. Certain of these detection techniques are again difficult to implement and are basically unreliable. It is therefore an object of the present invention to provide a fault detector in a dual intrusion type detection system which is simple and economical to employ.
It is a further object of the present invention to provide a supervision circuit to enable the user of the system to determine failure of various system components to thereby also warn the user of a marginal condition.